Generally marine pipelaying vessels are equipped with an A&R system, which is an abbreviation of “abandonment and recovery system”.
An example of a marine pipelaying vessel equipped with an A&R system is disclosed in WO2007/108673. This known vessel comprises a pipeline launch tower. A pipeline diverter is supported at an elevated position on the tower. Below the pipeline diverter there are retractable tensioners supported by the tower. A hang-off clamp allows to hold the seagoing pipeline when it has been severed from the pipeline section that is held by the tensioners. This known vessel also has an A & R system for abandonment and recovery of a pipeline. The A&R system comprises an A&R winch and A&R cable, as well as an arrangement of sheaves for this cable including an A&R cable suspension sheave that is displaceable mounted on the tower and allows to suspend the A&R cable in the firing line.
Whilst not specifically disclosed in WO2007/108673 it is common practice to provide such vessel with an initiation system for initiation of the pipelaying process. A known initiation system comprises an initiation winch and initiation cable. In an initiation procedure the initiation cable is passed—via an initiation cable sheave arranged generally underneath the lowermost tensioner—upward through the tensioners and then over the pipeline diverter, so that this cable can be connected to the pipeline to be laid. This pipeline is for example spooled onto a horizontal axis storage reel or in a vertical axis carousel storage. By operation of the initiation winch, which has a significant lower traction capacity than the A&R winch, the pipeline is pulled over the pipeline diverter and into the tensioners. Then the tensioners are made to engage on the pipeline and the initiation cable is disconnected.
A common A&R procedure, e.g. as possible with the vessel of WO2007/108673, comprises the steps of:                halting the pipelaying and engaging the hang-off clamp with the pipeline at a position below the one or more tensioners,        transferring the weight of the seagoing pipeline onto the hang-off clamp,        severing the pipeline between the hang-off clamp and the one or more tensioners,        connecting the A&R cable to the upper end of the seagoing pipeline held by the hang-off clamp,        releasing the hang-off clamp,        operating the A&R winch and thereby lowering the pipeline onto the seabed.        
As water depths at which pipelaying is performed are very significant, with depths of over 1000 meters being no exception and depths in the range of 2000 meters and 3000 meters becoming more and more common, the requirements placed on A&R systems and corresponding A&R procedures are ever increasing.
For instance the major water depths cause the need for very significant load capacities of the A&R system at great depth.
A further factor is the speed and ease at which the abandonment procedure can be performed, e.g. the requirements on personnel involved and the time required to perform the abandonment in view of worsening weather conditions.
Yet another factor is the effect that the A&R procedure may have on the pipeline section that remains in the pipeline installation. For example in some known A&R procedures it is envisaged that the pipeline section which is held in the tensioners at the time of halting the pipelaying is discarded in the course of the abandonment procedure.
It has been found that existing A&R procedures and systems are not entirely satisfactory.